Asphalt or flexible pavement is typically built with several layers to form a layered system with better materials at the top where the stress intensity is high and inferior materials at the bottom where the stress intensity is low. The top layer, called the surface course, is typically made of an asphalt mixture. All types of failure or distress can be classified by whether they are structural or functional failures and load associated or non-load associated distresses. Surface course aging is considered a non-load associated distress caused by climate/environmental effects. Many environmental factors can cause surface course aging damage, such as ozone, UV rays, oxygen, and thermal radiation. Oxidative aging causes the asphalt binder to become harder and more brittle. Aging can be characterized as two parts, short-term aging and long-term aging.
Most of the short-term aging that occurs in asphalt begins with the blending of the aggregate with asphalt binders. The blending temperature in the asphalt plant primarily controls the oxidative aging rate of the asphalt. The short-term aging for the asphalt binder in the mixture continues until the end of the pavement construction. Methods such as warm mix asphalt and cold mix asphalt are the main solutions to reduce the short-term aging via heating and constructing the asphalt mixture at lower temperatures compared with hot mix asphalt.
During the service life, the long-term oxidative aging begins and it occurs at a much slower rate than the rate of aging during mixing and construction. The brittleness of the asphalt mixture gradually increases due to physic-chemical changes in the binder. Exudation, evaporation, oxidation, and physical aging are all related to asphalt binder aging, while oxidation and physical hardening (steric hardening) are the most important direct consequences. Physical aging is a reversible process, which can produce changes in rheological, electrical and caloric properties, etc., without altering the chemical composition of the material. The oxidation of asphalt binder caused by chemical reactions causes transformations in the asphalt components. Asphalt oxidation is the main cause of long-term deterioration and eventually results in cracking in asphalt pavements. The asphalt can be separated into four generic fractions namely: asphaltenes, polar aromatics, naphthene aromatics, and saturates. Each fraction provides different properties. Asphaltenes mainly contribute to the viscosity (hardening effect), and the aromatics and saturates are correlated to the ductility (elastic effect). Many researchers have compared the fractions of aged asphalt with fractions of unaged asphalt. It was found that the oxidation of asphalt had an effect on chemical properties and consequently on the rheological properties. While asphalt is aging, the viscosity increases due to the oxidative conversion of polar aromatics to asphaltenes. This transformation between the components during oxidation can be described as follows: Aromatics→Resins→Asphaltenes. The polymerizations or condensations of the asphaltenes create larger molecules with long chained structures and affect the hardening of the asphalt. The oxidation causes a great increase in the asphaltenes and a larger amount of asphaltenes with high molecular weight, as evident by correlations with asphalt hardening. This asphalt hardening theory can be used to explain a condition known as the air-blown asphalt phenomena. An antioxidant additive is added to stop or delay the oxidative processes that convert aromatic fractions.
Common asphalt additives, such as polymers, rubber, and plastic do not prevent oxidative aging of the binder. The polymer additives can react with the free radicals but then degradation occurs to the additives. The polymers and other rubbers cannot prevent the propagation of free radical or peroxy radicals, because the polymer degradation can form new free radicals and the cycle repeats.
Research has been done by adding chemicals as antioxidant additives. No additive has been successful due to the environmental and economic concerns. Lead diamyldithiocarbamate (LDADC) does have an antioxidant effect, but it contains lead and is not an environmentally friendly material. Zinc kailkyldiathiophashate (ZDDP) and zinc diabutyldithiocarbamate (ZDBC) have been tested and proven to have antioxidant effects in asphalt binder, but their application can be limited by cost considerations.
The present invention is directed to overcoming these and other deficiencies in the art.